A positive electrode active material having an α-NaFeO2 structure such as a lithium cobalt oxide and a lithium nickelate can increase a chargeable and dischargeable capacity by increasing a charging voltage. However, when a battery is used in a high charging voltage region, an unintentional Li insertion and separation reaction of the positive electrode active material occurs due to a locally higher potential state.
According to Non-Patent Literature 1, when charging and discharging are performed at a high potential, Li insertion and separation that are necessary for crystalline phase transition are used. A reversible reaction of a Li insertion and separation reaction necessary for the phase transition is difficult and unstable, and causes capacity deterioration when a part of a positive electrode is exposed to a high potential according to a local potential distribution.
Patent Literature 1 discloses a technology for improving a cycle characteristic using an amorphous LiCoO2 as a positive electrode active material. However, in this method, it is not possible to use Li insertion and separation necessary for crystalline phase transition and a capacity significantly decreases. In addition, many side reactions occur at a high potential and particularly elution of a metal such as Co occurs frequently.
Patent Literatures 2 and 3 disclose technologies for reducing a movement resistance of lithium ions between positive electrode powders and improving a cycle characteristic by covering at least a part of a surface of a powder body with an amorphous lithium transition metal oxide. However, in these methods, while a movement resistance of lithium ions can be reduced, since it is not possible to ensure reversibility of crystalline phase transition, a capacity decreases when a battery is used at a high potential. In addition, many side reactions occur at a high potential and particularly elution of a metal such as Co occurs frequently.